There has been a phase-locked loop (PLL) apparatus outputting an electric signal having a phase synchronized with a reference electric signal. When an output frequency is a high frequency, a phase-locked loop apparatus 200 of an analog PLL as shown in FIG. 18. The phase-locked loop apparatus 200 is configured to include a signal source 210, an electric mixer 220 as a phase detector, a loop filter 230, a voltage controlled oscillator (VCO) 240 and a pre-scaler 250.
The operations of the electric mixer 220 as a phase comparator are shown in FIG. 19. As shown in FIG. 19, the electric mixer 220 outputs a product of an input electric signal as a reference signal from the signal source 210 and the output electric signal which has been output from the VCO 240 and has been subjected to frequency dividing by the pre-scaler 250. Consequently, the difference frequency and the sum frequency of the two signals are output in the frequency domain. Because the difference frequency component reflects the phase difference between the phases of the two signals, the electric mixer 220 is able to operate as the phase comparator by extracting only the difference frequency with the loop filter 230. The loop filter 230 outputs a positive signal, 0 signal and a negative signal when the phase difference between the input electric signal and the output signal of the VCO 240 is 0 degree, 90 degrees and 180 degrees, respectively, and the signal continuously changes according to the phase difference.
In this manner, the analog PLL performs the negative feedback control of the VCO so that the output signal of the phase comparator becomes 0, and the output phase of the VCO synchronizes with the phase of the input electric signal in the state of being shifted by 90 degrees consequently. However, the phase-locked loop apparatus which outputs a signal of a frequency of 100 GHz or more has the following difficult points:
1. it is difficult to manufacture the electric mixer for a signal of 100 GHz or more, and
2. it is difficult to manufacture the VCO capable of outputting an electric signal of 100 GHz or more.
Consequently, the phase-locked loop apparatus outputting an electric signal of 100 GHz or more is difficult to manufacture.
On the other hand, there is an optical phase-locked loop apparatus as a phase-locked loop apparatus capable of outputting a signal of 100 GHz or more (see, for example, Patent Document 1) Now, an example of the optical phase-locked loop apparatus will be described, referring to FIG. 20.
As shown in FIG. 20, an optical phase-cocked loop apparatus 300 is configured to include three elements of a phase detector composed of a multiplexer 310 and a phase comparator 320, a loop filter 330 and an optical voltage controlled oscillator (OVCO) 340. The optical phase-locked loop apparatus 300 receives a light signal as an input signal, and outputs a beat light signal synchronizing with the input signal from the OVCO 340.
The OPLL 340 is composed of laser diodes (LD's) 341 and 342, and an optical coupler 343. The OVCO 340 multiplexes the output lights of the two LD's 341 and 342 having output light wavelengths different from each other with the optical coupler 343. The polarizations of the optical outputs of the LD's 341 and 342 are made to be the same hereupon As a result, the two optical outputs interfere with each other in the optical coupler 343, and the optical coupler 343 outputs a beat light oscillating at a frequency according to the difference between the wavelengths. The restriction of the wavelength difference is determined by the band of the optical coupler 343. Because an ordinary optical coupler has a band of 10 THz or more, the ordinary optical coupler is able to easily output a beat light oscillating at a frequency of 100 GHz or more. Moreover, the LD's 341 and 342 are able to change the output wavelength thereof according to a drive current and a controlled temperature. Consequently, the OVCO 340 is able to control the output wavelengths of the LD's 341 and 342 according to an input light signal, and is able to change the frequency of the beat light signal to be output.
The phase comparator 320 outputs an electric signal according to the phase difference between the light intensities of two input signals using a nonlinear effect such as two-photon absorption, second harmonic wave generation, four-wave mixing, Kerr effect or the like. There are an avalanche photodiode (APD) a photon multiplier tube (PMT) and the like as the devices performing the two-photon absorption; there are a nonlinear crystal, a periodical poled lithium niobate (LiNbO3) crystal (PPLN) and the like as the second harmonic wave generation devices; there are a highly nonlinear fiber, a semiconductor optical amplifier (SOA) and the like as the four-wave mixing devices; and there is a nonlinear optical loop mirror (NOLM) as a method of using the Kerr effect. A nonlinear phenomenon happens in a short time less than several hundreds fs, and has a frequency band of 10 THz or more. Consequently, phase comparison is able to be performed to the signals of 100 GHz or more.
For example, a Si-APD has little sensitivity to the light of the wavelength of 1550 nm, however, has certain sensitivity to the light of the wavelength shorter than 800 nm. Consequently, any photoelectric currents are hardly generated when the light of 1550 nm enters the Si-APD, however, the Si-APD has certain sensitivity to the two-photon absorption phenomenon of 1550 nm. By installing an optical system so as to have the focus thereof on the surface of the Si-APD, the probability of the occurrence of the two-photon absorption becomes larger, and a photoelectric current that is exceptionally larger in comparison with the case of radiating light on the whole Si-APD is generated. The photoelectric current generated by the two-photon absorption in the Si-APD has a tear in proportion to the cosine of the mutual phase difference φ (cos φ) between two input lights. The sum of a steady-state value that is determined by optical power regardless of the phase difference φ and the part caused by the contribution of the cosine cos φ is output as the photoelectric current generated by the two-photon absorption. The contribution of the cosine cos φ is able to be extracted as the difference between an output photoelectric current and an average photoelectric current, and is able to be used as a phase comparison signal.
The loop filter 330 forms the phase comparison signal and outputs a control signal of the OVCO 340, as it is used in an ordinary phase-locked loop apparatus.
The operation of optical phase-locked loop apparatus 300 as the optical phase-locked loop apparatus is similar to that of the analog phase-locked loop apparatus of FIG. 18 except that the OVCO 340 outputs a beat light signal, and except that the optical coupler 310 and the phase comparator 320 uses the nonlinear effect to perform the phase comparison between an input light signal and the beat light signal. By the optical phase-locked loop apparatus 300, the signal generation in which a beat light synchronized with an input light signal, which beat light is 100 GHz or more, is able to be output becomes possible.
Moreover, an electric phase-locked loop apparatus the electric phase comparator of which is replaced with an optical phase comparator using the two-photon absorption in a Si-APD was also considered (see, for example, Non-patent Document 1). In Un-patent Document 1, an electric signal synchronized with the light signal of 12.5 Gbits/s is output.    [Patent Document 1] International Publication Pamphlet No. 03/104886    [Non-patent Document 1] Reza Salem, T. E. Murphy “Broad-Band Optical Clock Recovery System Using Two-photon Absorption”, IEEE Photon. Technol. Lett., vol. 9, pp. 2141-2143, Sep. 2004.